The present invention relates to a process for preparing crystalline aluminosilicate zeolites.
Crystalline aluminosilicate zeolites (hereinafter referred to simply as zeolites) have as a basic skeletal structure thereof a three-dimensional network of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of oxygen atoms to the total aluminium and silicon atoms, i.e. O/(Al+Si), is equal to 2. Therefore, the negative electrovalence of the tetrahedra containing aluminium is electrically balanced by the inclusion in the crystal of a cation, for example, an alkali metal ion, an alkaline earth metal ion, an ammonium ion, or a hydrogen ion. It is well known, as one of most important properties of zeolites, that these cations can be replaced with other cations by a conventional ion exchange method.
It is also well known that the crystal of zeolites has a large number of small cavities of the molecular order, which cavities are interconnected by a number of still smaller channels. Generally, these small cavities are occupied by water of hydration. After this water of hydration is dehydrated at least partially under suitable conditions, other molecules can be adsorbed and held within those small cavities. In other words, zeolites have a characteristic as an adsorbent. The size and shape of molecules which can be adsorbed are restricted by the dimensions of those pores. Consequently, it becomes possible to effect the so-called adsorptive separation in which some particular molecule is separated from a mixture which contains the molecule on the basis of the size or shape thereof. Furthermore, in addition to the size and shape of molecule, there are factors which permit a selective adsorption of a certain kind of molecule. For example, these factors include the polarizability and the degree of unsaturation of the adsorbed molecule, as well as the polarizing force, the size of cations and the degree of hydration within the pores of the zeolites. With these factors it is also made possible to effect a selective adsorption.
Another characteristic feature of zeolites is their high catalytic activity. Particularly, by replacing the ion-exchangeable alkali metal ions of zeolites with ammonium ions, hydrogen ions, or polyvalent metal cations, e.g. rare earth metal ions, a solid acidity is developed, which exhibits a high catalytic activity in many reactins.
Zeolites generally include natural and synthetic zeolites. Examples of natural zeolites include analcite, natrolite, heulandite, clinoptilolite, phillipsite, mordenite, chabazite and faujasite. Examples of synthetic zeolites include zeolites A, B, D, E, F, G, H, J, L, M, Q, R, S, T, U, X, Y, and Z.
In general, natural zeolites contain many amorphous substances or other heterogeneous zeolites or crystals which are not zeolites, such as feldspar and quartz, and their crystallinity is low. Usually, due to the presence of these impurities, the pores of zeolites are closed and the zeolites cannot fully exhibit the advantageous characteristics.
In the case of synthetic zeolites, purity can be made extremely high, and the pore diameter uniform, so that synthetic zeolites have superior characteristics as adsorbents or catalysts as compared with natural zeolites. For this reason, a large number of zeolites have so far been prepared, part of which are industrially produced and used in many applications.
Generally, zeolites are produced by preparing an aqueous reaction mixture containing a silica source, an alumina source, an alkali source and/or an alkaline earth metal source and crystallizing it under reaction conditions under which the zeolites can be prepared.
However, many of the conventional synthetic zeolites are relatively low in the silica to alumina ratio. In general, there is the tendency that as the silica to alumina ratio becomes higher, the catalytic activity and the heat resistance become higher as well. The preparation of zeolites having a high silica to alumina ratio is disclosed, for example in U.S. Pat. No. 3,702,886, in which an organic quaternary ammonium is added to the zeolite preparing system. However, the organic quaternary ammonium has drawbacks such as its expense. It also emits an offensive odor and thus is difficult to handle and corrodes the apparatus in which it is used. The use of a quaternary ammonium is further diadvantageous in that quaternary ammonium ions are incorporated as cations in the zeolite prepared and then it becomes difficult to exchange into desirable cations.